1. Field of the Invention
This invention relates to a method of determining the position of a signal reception point by receiving and processing radio wave signals transmitted from satellites on geosynchronous orbits.
2. Prior Art
As conventional radio wave navigation systems, such as Decca, Omega and Loran-C which have oscillation sources on the earth are known. NNSS (Navy Navigation Satellite System) and GP (Global Positioning System), both using satellites are also known. The systems using satellites allow the basis of position determination to be expanded to cosmic space so that enhanced accuracy in position measurement ca be expected by the increased range of position detection and the provision of high grade equipment. The GPS is a type of system that measures range by time and has many superior performance characteristics in comparison with NNSS which uses the Doppler effect, and is going to be a main system for navigation satellites in the future. The prior art will further be described with reference to such GPS in order to compare the present invention therewith.
FIG. 1 illustrates the disposition of GPS satellites. NAVSTAR (Navigation System with Timing and Ranging) satellites 300, 301, 302, and 303 are at positions A', B', C' and D'. Each of the satellites has an accurate atomic clock to precisely maintain the accuracy of clock signals generated thereby. The atomic clock is calibrated by a signal sent from a control station on the earth so a to accurately tell the current time on the basis of the beginning of a week. The positions of NAVSTAR satellites are determined by the time at a specified instance since the control station determines their orbits from the tracking data of the satellites. Thus, if an observer were at the position of a NAVSTAR satellite, the current time of the satellite and the orbit elements serve to determine the position of the observer. Then, the case where the observer observes the satellite at a position away therefrom is considered, the observation point being designated by P'. Assume that the observer has a clock with no calibrated high accuracy and with a fixed error from a reference time, and that when clocks on the satellites point to a time Tn.sub.o, the clock of the observer points to a time (Tn.sub.o +.DELTA.t), where .DELTA.t is an inherent error in the observer. Satellites 300, 301, 302, 303 are assumed to be measured by the observer at times Tn.sub.1, Tn.sub.2, Tn.sub.3 and Tn.sub.4, respectively. Taking into consideration the propagation time between the observer and the satellites, the following equations are obtained. EQU A'P'=C(Tn.sub.0 +.DELTA.t -Tn.sub.1) EQU B'P'=C(Tn.sub.0 +.DELTA.t Tn.sub.2) EQU C'P'=C(Tn.sub.0 +.DELTA.t-Tn.sub.3) EQU D'P'=C(Tn.sub.0 +.DELTA.t Tn.sub.4)
where, C is the velocity of light.
The above Equations have, as their unknown factors, .DELTA.t and three three-dimensional values defining the position P'. The Equations thus have solutions to enable the observer's position and time to be calibrated.
The NAVSTAR satellites are on six circular 12-hour orbits at the altitude of 20 degree, 183 km, the orbit inclination being at 55 degrees. Three satellites are positioned at an equal distance on each of such six orbits and totally 18 satellites are moving in the six orbits. The positions of the satellites are determined such that four of them are always visible from any point on the earth.
One control station and at least four monitoring stations are provided on the earth. Each station receives data and transmits necessary commands when the four satellites are in the visible range. Processing of the orbit data is performed by data processing equipment on the basis of the obtained data, and the time of the calibration is practised on the basis of information from the primary reference.
GPS requires at least 18 NAVSTAR satellites to enable position measuring at all points on the earth, and also requires highly accurate clocks and frequencies for generating original data for range measurement. Thus they must be equipped with costly atomic clocks in order to maintain a measurement accuracy for a long period so that the control station can calibrate them once a week to maintain changes in time and frequencies within allowable ranges.